Diesel vs. Gasoline: Is Diesel Worth the Added Cost?

Folks are often distracted by the cost-differential between regular unleaded gasoline and diesel fuel. I can’t count the number of times I’ve heard variations of the same question, “how can you justify buying a diesel-powered car with the 30 cent difference between diesel and gasoline?”

Diesel engines can be as much as 30% more fuel efficient than the equivalent gasoline engine. The cost-differential between diesel fuel and regular unleaded varies depending on factors that include seasonal fluctuations, geographic location, and market demand.

So how do diesel cars save money over their gasoline engine counterparts, when diesel fuel is more expensive than regular unleaded gasoline?

Do the math and it becomes apparent. That thirty cent per gallon difference can be quickly eclipsed.

We’ll compare two hypothetical cars – both the same make and model – one with a conventional gasoline engine that gets 30 MPG and one with a diesel engine that gets 40 MPG. Lets say that regular unleaded is $4.00 per gallon and ultra-low sulfur diesel fuel is $4.30 per gallon. Over 20,000 miles the total fuel cost for the gasoline-powered car is $2,660, compared to a total fuel cost for the diesel-powered car of $2,160. That works out to a savings of $500 per year, or 0.025 cents per mile.

You will not get a true reading when comparing disparate cars. It’s crucial to compare apples-to-apples. This is done most easily with Volkswagen’s vehicles, which offer a range of engine choices.

The gasoline-powered 2012 VW Jetta is officially rated at 24 city / 34 highway, while the TDI version is rated at 30 / 42. In this case, the city figure represents a 25% improvement. If you take driveability into consideration, the numbers are more dramatic. The gas Jetta delivers 115 horsepower (HP) and 125 foot-pounds of torque, while the diesel version cranks out 140 HP and 236 foot-pounds of torque. Get behind the wheel and you’ll feel the difference.

So now lets compare apples to oranges …

Pit the VW Jetta against the Mazda3 Skyactiv, and it’s a whole new story, with the Skyactiv delivering 28 city / 40 highway, as well as a respectable 155 HP and 148 foot-pounds of torque. But it’s not just about engine technology. Advanced aerodynamics are key to the Skyactiv 3’s success and Mazda isn’t content with the gasoline engine’s current fuel economy figures. They’ll be bringing a diesel version – the SkyactivD – to the States in the coming years and we expect the diesel’s highway MPG ratings to rise into the fifties.

It’s important to consider residual value, as well. Diesel vehicles may cost more initially, but they historically maintain a much higher resale value, due to the longevity of the engines.. When you look at the total cost of ownership – from purchase and maintenance, through fueling and residual value – diesel versions tend to be more frugal than their gasoline-powered counterparts over the long run.

Keep in mind that the price of diesel fuel rises heading into winter and drops through the spring, due to the demand for heating oil. Diesel fuel and heating oil are nearly one and the same, with the biggest difference being the colorant that’s added to road fuel. (Diesel fuel has road taxes, while heating oil does not, and the government doesn’t want you to run untaxed heating oil in your diesel vehicle, lest they lose revenue.)

Geography matters, as well. In some parts of the country, diesel fuel is disproportionally more expensive. In other parts, far less so. Much of this can be traced to supply and demand, as it relates to refinery output and local consumption. World demand for diesel fuel can have a notable effect. As economies heat up, America’s refineries produce a significant amount of fuel for export.

To sum things up, you need to apply your own factors to accurately gauge whether a gasoline or diesel-powered vehicle is the most cost-effective choice for your individual requirements. Don’t forget to include torque, longevity, and residual value in the equation.